JP5297657B2 - Active silencer system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active noise eliminating system, optionally and freely selecting a frequency band to be eliminated from noise, and canceling only the sound of selected frequency band. <P>SOLUTION: This active noise eliminating system 10A includes: a filter 21 generating a pseudo feedback signal, which estimates a second sound wave reaching from a speaker 13 to a microphone 11; a subtraction device 22 generating a third input signal obtained by removing a pseudo feedback signal from a first input signal; a target signal generating device 24 for controlling the frequency characteristic of a third input signal to convert the third input signal to a target signal having a predetermined frequency band and sound pressure level; and a subtraction device 25 generating an error signal obtained by removing the target signal from the second input signal, an interference signal generating device 23 generates an interference signal using the third input signal received from the subtraction device 22 and an error signal received from the subtraction device 25. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to an active silencing system that cancels noise propagating in a predetermined space.

  There is a silencing system that generates a sound wave for silencing with an antiphase sound pressure with respect to noise propagating in the duct, and cancels the noise in the duct by causing the noise and the sound wave for silencing to interfere (see Patent Document 1). . This silencing system is arranged between a first sensor for detecting noise in the duct, a second sensor for detecting the silencing state in the duct, and the first and second sensors, and has the same sound pressure as that of the noise. Formed from a speaker that generates a sound wave for silencing, a signal generation device for silencing that outputs a sound signal to be silenced to the speaker, and a frequency cutoff filter that blocks signals in the middle to high frequency band that are not to be silenced Has been.

The silencing signal generator is formed of an adaptive control arithmetic unit and a silencing signal generating filter. The adaptive control arithmetic unit updates the mute signal generation filter. The mute signal generation filter generates a mute signal that minimizes the output signal of the second sensor. Since this silencer system uses a frequency cutoff filter by digital signal processing that blocks signals in the middle to high frequency band that are not subject to mute, it can instantly and accurately intercept the signal in the middle to high frequency band. Thus, the low frequency band signal to be muffled can be reliably extracted. Further, by using a frequency cutoff filter based on digital signal processing, it is possible to shorten the time delay when performing the silencing process.
Japanese Patent Laid-Open No. 7-334168

  The silencing system disclosed in Patent Document 1 shuts off a medium to high frequency band signal that is not to be silenced, and silences a low frequency band using a sound wave for silencing with an antiphase sound pressure. Since it is not possible to freely select a frequency band to be silenced in the low frequency band and all the sounds in the low frequency band are silenced, noise in the duct may be erased more than necessary. If the noise in the duct is extinguished more than necessary, the voice of the occupants in the room using the duct, the sound of TV, stereo, etc. will sound in various places in the room, causing discomfort to the resident, Speech privacy may not be protected.

  Also, when using the noise in the duct as masking noise for speech privacy protection, if you want to keep the sound in the frequency band of the noise without reducing the sound in the frequency band of the speech. There are times when you want to raise the volume of the frequency band. However, this silencer system cannot select such a frequency band and increase or decrease the volume of that frequency band.

  An object of the present invention is to provide an active silencing system capable of arbitrarily and freely selecting a frequency band to be silenced from noise and canceling only the sound of the selected frequency band. Another object of the present invention is to reduce the volume of the frequency band without completely muting the sound of the predetermined frequency band, or to increase the volume of the frequency band. An object of the present invention is to provide an active silencing system in which the frequency band can be arbitrarily and freely selected, and the volume of the selected frequency band can be increased or decreased.

  The premise of the present invention to solve the above-mentioned problems is that the first sensor arranged at a predetermined distance from the noise source and the predetermined distance from the noise source across the first sensor are generated from the noise source. A second sound wave generating device that generates a second sound wave that interferes with the first sound wave propagating in the predetermined space in the space, and a second sound wave device disposed at a predetermined distance from the first sensor across the second sound wave generating device. Two sensors and an interference signal generating device that generates an interference signal to be a second sound wave and outputs the generated interference signal to the second sound wave generating device, and the first sensor has a first sound wave and a second sound wave. The synthesized sound wave is detected in time series, and the detected synthesized sound wave is output as a first signal, and the second sensor detects and detects the interference sound wave generated by the interference between the first sound wave and the second sound wave in time series. An active silencer system that outputs interfering sound waves as a second signal. That.

The first feature of the present invention based on the premise is that the active silencer system generates a pseudo feedback signal generation filter that generates a pseudo feedback signal obtained by estimating the second sound wave reaching the first sensor from the second sound wave generator, A first subtracting device that receives a pseudo feedback signal from a pseudo feedback signal generation filter while generating a first signal from the sensor and generates a third signal obtained by removing the pseudo feedback signal from the first signal; Receiving a third signal, adjusting a frequency characteristic of the third signal and converting the third signal into a target signal having a predetermined frequency band and sound pressure level; and a second signal from the second sensor. A second subtracting device that receives the target signal from the target signal generating device and generates an error signal obtained by removing the target signal from the second signal. Forming apparatus, a first digital filter, the volume of the frequency band to be mute or attenuate of arbitrarily selected frequency band of the third signal minus simulating the time series transfer characteristic from the first sensor to the second sensor And an equalizer that sets the volume of a frequency band desired to be amplified or remaining in an arbitrarily selected frequency band of the third signal to a plus, and a first digital filter preliminarily estimated by the first digital filter The time series transfer characteristic in the noise transmission path from the first sensor to the second sensor is used, the time series transfer characteristic is convolved with the third signal, and the time delay until the first sound wave propagates from the first sensor to the second sensor is calculated. While correcting, the frequency characteristics of the first sound wave are matched with those of the first sound wave at the position of the second sensor, so that the characteristics of the noise transmission path are changed to the third signal. In a state that reflects the, outputs the third signal to the equalizer, the interference signal generating device, the third signal and the interference signal by using the error signal received from the second subtractor which receives from the first subtractor Is to generate.

The second feature of the present invention based on the above premise is that the active silencer system generates a pseudo feedback signal generation filter that generates a pseudo feedback signal that estimates the second sound wave that reaches the first sensor from the second sound wave generator, A first subtracting device that receives a pseudo feedback signal from a pseudo feedback signal generation filter while generating a first signal from the sensor and generates a third signal obtained by removing the pseudo feedback signal from the first signal; Receiving a third signal, adjusting a frequency characteristic of the third signal and converting the third signal into a target signal having a predetermined frequency band and sound pressure level; and a second signal from the second sensor. A second subtracting device that receives the target signal from the target signal generating device and generates an error signal obtained by removing the target signal from the second signal. A delay correction device for correcting a time delay until the first sound wave propagates from the first sensor to the second sensor, and a frequency band in which the third signal is desired to be silenced or attenuated in an arbitrarily selected frequency band; And an equalizer for setting the volume of the frequency band desired to be amplified or remaining in the arbitrarily selected frequency band of the third signal to be plus, and the delay correction device includes the first sensor. By correcting the time delay of the first sound wave from the second sensor to the second sensor, the third signal is output to the equalizer with the time delay reflected in the third signal, and the interference signal generating device An interference signal is generated using the third signal received from the subtracting device and the error signal received from the second subtracting device.

As an example of the present invention , the amplified target signal is output from the target signal generating device to the interference signal generating device via the second subtracting device in the frequency band in which the volume is set to be positive in the equalizer, and at the position of the second sensor. In the frequency band in which the second sound wave generator outputs the second sound wave so as to amplify the sound and the volume is set to be negative in the equalizer, the attenuated target signal is transmitted from the target signal generation device via the second subtraction device. The second sound wave generation device outputs the second sound wave so as to attenuate the sound at the position of the second sensor that is output to the interference signal generation device .

  As another example of the present invention, an interference signal generation device uses an adaptive digital filter for generating an interference signal that minimizes the second signal in time series, and an adaptive digital signal using an error signal received from the second subtraction device. An adaptive control arithmetic unit that updates the filter in time series is formed.

  As another example of the present invention, the interference signal generation device includes a second digital filter that simulates time-series transfer characteristics from the second sound wave generation device to the second sensor, and the adaptive control arithmetic device includes the second digital filter. The adaptive digital filter is updated in time series based on the transfer function at and the error signal received from the second subtractor.

  As another example of the present invention, the sampling frequency in the active silencing system is in the range of 3 kHz to 40 kHz.

  As another example of the present invention, the space is an internal space of the air conditioning duct.

  According to the active silencing system of the present invention, the target signal generation device generates a target signal having a desired frequency characteristic after interference with the second sound wave, and the second subtraction device removes the target signal from the second signal. Since the error signal is generated, the second signal output from the second sensor can be approximated to the target signal, and the target signal generator can arbitrarily and freely set the frequency characteristic that the frequency characteristic of the position of the second sensor is expected. Can be adjusted to. This active silencing system outputs only the frequency band to be silenced from the second signal as an error signal to the interference signal generation device, and the interference signal generation device generates an interference signal using the error signal and the third signal. An interference signal that interferes with only the sound of the frequency band to be silenced can be output to the second sound wave generating device via the interference signal generating device. Therefore, it is possible to arbitrarily and freely select a frequency band to be silenced from noise propagating in a predetermined space, and it is possible to surely cancel only the sound of the selected frequency band. This active silencing system can arbitrarily and freely select the remaining frequency band to remain from the second signal, and can increase or decrease the volume of the remaining frequency band. It can be used as masking noise for protection.

  This active silencing system generates a third signal obtained by removing a pseudo feedback signal that can be estimated as a second signal from the synthesized signal by the first subtractor, and outputs the third signal to the interference signal generator and the target signal generator. Therefore, the third signal output to the interference signal generation device or the target signal generation device does not include the pseudo feedback signal that estimates the second sound wave reaching the first sensor, and prevents howling in the first sensor. Can do. When the combined signal including the interference signal is output to the interference signal generation device as it is, an interference signal including a sound wave that silences the interference signal is generated. In this system, the first signal obtained by removing the pseudo feedback signal from the combined signal is generated. Since the three signals are output to the interference signal generation device, it is possible to prevent the interference signals from including sound waves that mute them. Further, when an interference signal is included in the signal output to the target signal generation device, a composite signal of the first signal and the interference signal is output to the target signal generation device, and it becomes difficult to select a frequency band to be silenced. However, in this system, since the third signal obtained by removing the pseudo feedback signal from the synthesized signal is output to the target signal generation device, it is possible to reliably select the frequency band to be silenced in the target signal generation device.

  In the active silencing system in which the target signal generating device is formed of the first digital filter and the equalizer, the time delay until the first sound wave propagates from the first sensor to the second sensor is corrected by the first digital filter. Since the frequency characteristic from the first sensor to the second sensor is used, the third signal is passed through the equalizer in a state where the characteristic of the noise transmission path from the first sensor to the second sensor is reflected in the third signal. And the target signal can be selected from the third signal via the equalizer under the assumption that the third signal is generated at the position of the second sensor. .

  In the active silencing system in which the target signal generation device is formed of the delay correction device and the equalizer, the time delay until the first sound wave propagates from the first sensor to the second sensor is corrected by the delay correction device. The frequency band and sound pressure level of the third signal can be adjusted via the equalizer with the time delay between the first sensor and the second sensor corrected, and the third signal is generated at the position of the first sensor. The target signal can be generated from the third signal through the equalizer in the state assumed to be.

  In the active silencer system in which the interference signal generation device is formed of an adaptive digital filter and an adaptive control arithmetic device, the adaptive control arithmetic device updates the adaptive digital filter using the error signal, and the interference digital signal is generated by the adaptive digital filter. Since it is generated, it is possible to reliably mute only the sound of the frequency band to be silenced by using the second sound wave generated from the second sound wave generating device while following the change of noise in the space and the change of the target signal. .

  In the active silencing system in which the interference signal generation device includes the second digital filter, the second digital filter corrects a time delay until the second sound wave propagates from the second sound wave generation device to the second sensor, and the second digital filter corrects the second delay time. Since the transmission frequency characteristic of the sound wave is used, the adaptive control arithmetic unit performs the adaptive digital filter using the error signal while considering the time delay and the frequency characteristic of the secondary path from the second sound wave generator to the second sensor. It is possible to update, and it is possible to reliably mute only the sound in the frequency band to be muffled at the position of the second sensor that is the mute position.

  An active silencing system whose sampling frequency is in the range of 3 kHz to 40 kHz can be as close as possible to the sound of the opposite phase and the same sound pressure of the sound that the second sound wave is to be silenced because the system samples at that frequency within a unit time. It is possible to mute only the sound of the frequency band to be silenced using the second sound wave.

  The active silencing system according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a configuration diagram of an active silencing system 10A shown as an example, and FIG. 2 is a diagram illustrating response characteristics. In FIG. 1, the longitudinal direction is indicated by an arrow X. The system 10A includes a reference microphone 11 (first sensor) and an error microphone 12 (second sensor), a mute speaker 13 (second sound wave generator), and a controller 14. The reference microphone 11, the error microphone 12, and the mute speaker 13 are connected to the controller 14 via an interface. The sampling frequency in this system is in the range of 3 kHz to 40 kHz, preferably in the range of 10 kHz to 40 kHz.

  The reference microphone 11 is disposed in the internal space 16 of the air conditioning duct 15 and is disposed at a position spaced apart from the noise source 17 of the duct 15 by a predetermined dimension in the longitudinal direction. The reference microphone 11 is connected to an amplifier 18 that amplifies the sound wave detected by the reference microphone 11. Although not shown, an A / D conversion device is connected between the controller 14 and the amplifier 18. The reference microphone 11 detects a synthesized sound wave of an analog first sound wave (noise) generated from the noise source 17 and propagating through the internal space 16 of the duct 15 and an analog second sound wave described later generated from the muffler speaker 13. Then, the detected synthesized sound wave is output to the amplifier 18. However, when the system 10A is activated, the reference microphone 11 detects only the first sound wave. The synthesized sound wave is amplified by the amplifier 18 and then converted into a digital first input signal (first signal) by the A / D converter. The first input signal is output from the A / D converter to the controller 14.

  The muffler speaker 13 is disposed in the internal space 16 of the air conditioning duct 15 and is disposed at a position spaced apart from the noise source 17 by a predetermined dimension in the longitudinal direction with the reference microphone 11 interposed therebetween. The muffler speaker 13 generates a second sound wave that interferes with the first sound wave in the internal space 16 of the duct 15. An amplifier 19 that amplifies the second sound wave is connected to the muffler speaker 13. Although not shown, a D / A converter is connected between the controller 14 and the amplifier 19.

  The error microphone 12 is disposed in the internal space 16 of the air conditioning duct 15 and is disposed at a position spaced apart from the reference microphone 11 in the longitudinal direction by a predetermined dimension with the muffler speaker 13 interposed therebetween. An amplifier 20 that amplifies the detected sound wave is connected to the error microphone 12. Although not shown, an A / D conversion device is connected between the controller 14 and the amplifier 20. The error microphone 12 detects an interference sound wave (error sound wave) generated by the interference between the first sound wave and the second sound wave, and outputs the detected interference sound wave to the amplifier 20. However, the error microphone 12 detects only the first sound wave when the system 10A is activated. The interference sound wave is amplified by the amplifier 20, and then converted into a digital second input signal (second signal) by the A / D converter. The second input signal is output from the A / D converter to the controller 14.

  The controller 14 includes a pseudo feedback signal generation filter 21, a first subtraction device 22, an interference signal generation device 23, a target signal generation device 24, and a second subtraction device 25. A central processing unit for executing various means in 22 to 25 and a memory for storing various data. The controller 14 uses a DSP (Digital Signal Processor) that processes convolution calculations at high speed, but a CPU (Central Processing Unit) or MPU (Micro Processing Unit) can also be used.

  The controller 14 generates a predetermined interference signal via the filter 21 and the devices 22 to 25, generates a second sound wave in the internal space 16 of the duct 15 via the muffler speaker 13, and Sounds in a predetermined frequency band are muted, or sounds in a predetermined frequency band are attenuated or amplified. Although not shown, the controller 14 is connected to an input device such as a keyboard and a mouse and an output device such as a display and a printer via an interface.

  The internal address file of the memory stores an application for causing various devices to execute each unit of the system 10A via the central processing unit. The central processing unit of the controller 14 activates an application from the memory based on the control by the operating system stored in the memory, and executes the following means according to the activated application. The central processing unit causes the interference signal generation device 23 to generate a digital interference signal (interference signal generation unit), and causes the pseudo feedback signal generation filter 21 to generate a digital pseudo feedback signal (pseudo feedback signal generation unit).

  The central processing unit of the controller 14 outputs the interference signal from the interference signal generation device 23 to the mute speaker 13 (interference signal output means), and outputs the pseudo feedback signal from the pseudo feedback signal generation filter 21 to the first subtraction device 22. (Pseudo feedback signal output means). The interference signal is a signal that is generated from the noise source 17 and propagates through the space 16 and becomes a second sound wave having an anti-phase sound pressure with a sound in a frequency band to be silenced or attenuated, and , A signal including a signal to be amplified and a second sound wave having the same homologous sound pressure. The pseudo feedback signal is a signal that can be estimated (identified) as the second sound wave that reaches the reference microphone 11 from the mute speaker 13.

  The central processing unit of the controller 14 causes the first subtractor 22 to subtract the pseudo feedback signal from the first input signal to generate a third input signal (third signal) (third signal generating means), and the third input thereof. The signal is output from the first subtractor 22 to the interference signal generator 23 and the target signal generator 24 (third signal output means). The central processing unit causes the target signal generating device 24 to generate a target signal having a desired frequency characteristic after interference with the second sound wave from the third signal (target signal generating means), and the target signal is output from the target signal generating device 24. The second subtracting device 25 outputs the signal (target signal output means). The central processing unit causes the second subtracting device 25 to subtract the target signal from the second input signal to generate an error signal (error signal generating means), and the error signal is transferred from the second subtracting device 25 to the interference signal generating device 23. Output (error signal output means).

  The pseudo feedback signal generation filter 21 simulates time-series transfer characteristics (Hr) from the mute speaker 13 to the reference microphone 11. The filter 21 reflects the time series transfer characteristic (Hr) (impulse response) between the muffler speaker 13 and the reference microphone 11 estimated in advance, and the first subtractor 22 generates a pseudo feedback signal from the first input signal. The feedforward control is executed on the first subtracting device 22 so that can be subtracted. The time series transfer characteristic 55 is a response characteristic at the sound receiving point 53 when the sound wave 51 (pulse) generated from the sound source 50 propagates through the transmission path 52 and reaches the sound receiving point 53 as shown in FIG. is there. That is, the time-series transfer characteristic (Hr) indicates that the second sound wave generated from the muffler speaker 13 propagates through the internal space 16 of the duct 15 and reaches the reference microphone 11 when the second sound wave at the position of the microphone 11 is reached. The response characteristics are shown.

  The interference signal generation device 23 is formed of a digital filter 26 (second digital filter), an adaptive digital filter 27 (adaptive digital filter), and an adaptive control arithmetic device 28 (Filtered-XLMS algorithm) that updates the adaptive digital filter 27. Has been. The digital filter 26 simulates time series transfer characteristics (C) from the muffler speaker 13 to the error microphone 12. In the filter 26, the adaptive control arithmetic unit 28 can update the adaptive digital filter 27 in a state in which the time-series transfer characteristic (C) (impulse response) between the muffler speaker 13 and the error microphone 12 estimated in advance is reflected. As described above, the feedforward control is performed on the adaptive control arithmetic unit 28. The time series transfer characteristic (C) is a response characteristic of the second sound wave at the position of the microphone 12 when the second sound wave generated from the muffling speaker 13 propagates through the internal space 16 of the duct 15 and reaches the error microphone 12. Is shown.

  The interference signal generation device 23 updates the adaptive digital filter 27 based on the adaptive control algorithm. The adaptive digital filter 27 generates an interference signal (signal that minimizes the second input signal) having the same sound pressure as that of the noise for canceling the noise in response to a change in the environment of the internal space 16 of the duct 15.

  The target signal generating device 24 is formed of a digital filter 29 (first digital filter) and an equalizer 30 (frequency characteristic adjusting device). The filter 29 simulates a time series transfer characteristic (C ′) from the reference microphone 11 to the error microphone 12. The filter 29 performs feed-forward control on the equalizer 30 based on the time-series transfer characteristic (C ′) (impulse response) of the noise transfer path from the reference microphone 11 to the error microphone 12 estimated in advance. The time-series transfer characteristic (C ′) shows the response characteristic of the first sound wave at the position of the microphone 12 when the first sound wave propagating through the internal space 16 of the duct 15 reaches the error microphone 12 from the reference microphone 11. Yes.

  The equalizer 30 converts the third input signal into a target signal having a desired frequency characteristic by adjusting the volume of the frequency band while selecting an arbitrary frequency band from the third input signal, and converts the target signal to the target signal. Output. As the equalizer 30, a 1-octave audio graphic equalizer, a 1/3 octave audio graphic equalizer, or the like can be used.

  An example of the silencing process in the system 10A of FIG. 1 will be described as follows. When an air conditioner (not shown) is started, a first sound wave (noise) propagates from the noise source 17 of the air conditioner to the internal space 16 of the air conditioning duct 15. When the system 10A is activated, the controller 14 causes the filters 21, 26, 27, and 29 and the devices 28 and 30 to execute various means according to the application stored in the memory. As shown by the arrow f, the first sound wave reaches the reference microphone 11 while propagating through the internal space 16 of the duct 15, passes through the muffler speaker 13, and reaches the error microphone 12.

  Note that as an advance preparation for executing various means in the system 10A, the equalizer 30 is adjusted to set the frequency band and the volume of the frequency band. In the frequency band where the volume is set to (+), the amplified target signal is output from the target signal generation device 24 to the interference signal generation device 23 via the second subtraction device 25, and as a result, at the position of the error microphone 12. The mute speaker 13 outputs the second sound wave so as to amplify the sound. In the frequency band where the volume is set to (−), the attenuated target signal is output from the target signal generating device 24 to the interference signal generating device 23 via the second subtracting device 25, and as a result, at the position of the error microphone 12. The muffler speaker 13 outputs the second sound wave so as to attenuate the sound.

  At the time of activation of the system 10A, the second sound wave is not generated from the muffling speaker 13, the first sound wave (noise) is detected by the reference microphone 11, and further propagates through the internal space 16 of the duct 15 to cause an error. The microphone 12 is reached, and the first sound wave is detected by the error microphone 12. The reference microphone 11 that has detected the first sound wave outputs the first sound wave to the amplifier 18. The amplifier 18 amplifies the first sound wave and then outputs the first sound wave to the A / D converter. The A / D converter converts the first sound wave into a digital first input signal, and then outputs the first input signal to the first subtractor 22. The error microphone 12 that has detected the first sound wave outputs the first sound wave to the amplifier 20. The amplifier 20 amplifies the first sound wave and then outputs the first sound wave to the A / D converter. The A / D conversion device converts the first sound wave into a digital second input signal, and then outputs the second input signal to the second subtraction device 25.

  Since the pseudo feedback signal generation filter 21 does not generate a pseudo feedback signal when the system 10A is activated, the output signal of the pseudo feedback signal output from the pseudo feedback signal generation filter 21 is (0). When the system 10A is activated, the first subtracting device 22 subtracts the pseudo feedback signal (0) from the first input signal to generate a third input signal (a signal having the same value as the first input signal) (third Signal generation means), and outputs the third input signal to the interference signal generation device 23 and the target signal generation device 24 (third signal output means).

  The digital filter 29 (first digital filter) forming the target signal generation device 24 uses the time series transfer characteristic (C ′) in the transmission path (noise transmission path) from the reference microphone 11 to the error microphone 12 estimated in advance. The time-series transfer characteristic (C ′) is convolved with the third input signal to correct the time delay until the first sound wave propagates from the reference microphone 11 to the error microphone 12, and the frequency characteristic of the first sound wave is changed to the error. It matches with that of the first sound wave at the position of the microphone 12. The filter 29 corrects the time delay and matches the frequency characteristics so that the characteristics of the noise transmission path are reflected in the third input signal, and the equalizer 30 that forms the target signal generation device with the third input signal. Output to.

  The equalizer 30 converts the third input signal into a target signal having a preset frequency characteristic (target signal generating means), and outputs the target signal to the second subtracting device 25 (target signal output means). In the equalizer 30, the volume of the frequency band desired to be muted or attenuated is set to (−), and the volume of the frequency band desired to remain or amplified is set to (+).

  When the system 10A is activated, the second subtractor 25 subtracts the target signal from the second input signal representing the first sound wave to calculate an error signal (error signal generator), and the error signal is used as the interference signal generator 23. (Error signal output means). Specifically, it is as follows. In the frequency band to remain, the volume of the equalizer 30 is set to (0). As a result, a signal having the same homologous amplitude as that of the second signal is output to the second subtracting device 25, and the error signal becomes (0). In the frequency band to be amplified, the volume of the equalizer 30 is set to (+). As a result, a signal having the same phase as the second signal and a large amplitude is output to the second subtracting device 25, and an error signal having a phase opposite to the phase of the second signal is output to the interference signal generating device 23. Conversely, in the frequency band to be attenuated, the volume of the equalizer 30 is set to (−). As a result, a signal having the same phase as the second signal and a small amplitude is output to the second subtracting device 25, and an error signal having the same phase as the phase of the second signal is output to the interference signal generating device 23. Further, in the frequency band to be silenced, the volume of the equalizer 30 is set to (−) maximum. As a result, the signal (0) is output to the second subtracting device 25, and an error signal having the same homologous amplitude of the second signal is output to the interference signal generating device 23.

  When the system 10A is activated, the value of the error signal, which is the difference between the target signal and the second input signal, is maximum. The digital filter 26 (second digital filter) forming the interference signal generation device 23 uses a time-series transfer characteristic (C) that simulates a response characteristic from the muffler speaker 13 to the error microphone 12 that is estimated in advance. The time series transfer characteristic (C) is convolved with the input signal, and the result is output to the adaptive control arithmetic unit 28 forming the interference signal generator 23.

  The adaptive control arithmetic unit 28 updates the adaptive digital filter 27 using the error signal while considering the time-series transfer characteristics of the secondary path. Specifically, the adaptive control arithmetic unit 28 generates a Δ adaptive digital filter (ΔADF) in time series at a predetermined time interval in accordance with a sampling frequency (3 kHz to 40 kHz), and the Δ adaptive digital filter is converted to the current adaptive digital filter ( By adding to (ADF), the adaptive digital filter 27 is updated from the present to the future.

  The interference signal generation device 23 performs a convolution operation on the adaptive digital filter 27 on the third input signal output from the first subtraction device 22, and generates an interference signal that minimizes the second input signal in time series (interference signal generation). means). The interference signal generation device 23 outputs the generated interference signal to the pseudo feedback signal generation filter 21 and the D / A conversion device (interference signal output means). The D / A converter converts the digital interference signal into an analog second sound wave, and outputs the second sound wave to the amplifier 19. The amplifier 19 amplifies the second sound wave and outputs the amplified second sound wave to the muffler speaker 13.

  The muffler speaker 13 generates a second sound wave in the internal space 16 of the duct 15. The second sound wave propagates from the speaker 13 toward the reference microphone 11 through the internal space 16 of the duct 15 and from the speaker 13 toward the error microphone 12 through the internal space 16. The second sound wave travels toward the error microphone 12 while interfering with the first sound wave, and completely interferes with the sound of the frequency band to be silenced or the sound of the remaining frequency band of the first sound wave at the position of the error microphone 12. While canceling the sound in the desired frequency band, it amplifies or attenuates the sound in the other frequency band. When the system 10A is continuously operated, the error signal gradually decreases, and finally the error signal level becomes (0) and the error disappears. The disappearance of the error represents the coincidence between the second signal that is the signal of the error microphone 12 and the target signal having the desired characteristic, and represents the state in which the signal of the error microphone 12 coincides with the desired frequency characteristic.

  During the operation of the system 10 </ b> A, the second sound wave toward the reference microphone 11 is synthesized with the first sound wave, and these synthesized sound waves are detected by the reference microphone 11. Further, the second sound wave traveling toward the error microphone 12 interferes with the first sound wave at the position of the microphone 12, and these interference sound waves are detected by the error microphone 12. The synthesized sound wave is amplified by the amplifier 18, converted to a digital first input signal by the A / D converter, and output to the first subtractor 22.

  The pseudo feedback signal generation filter 21 uses a time-series transfer characteristic (Hr) simulating a transmission path (tertiary path) from the muffler speaker 13 to the reference microphone 11 estimated in advance, and the time-series transfer characteristic (Hr) is used as an interference signal. ) To generate a pseudo feedback signal (pseudo feedback signal generating means). The pseudo feedback signal generation filter 21 outputs the generated pseudo feedback signal to the first subtraction device 22 (pseudo feedback signal output means). The first subtracting device 22 subtracts a pseudo feedback signal that can be estimated as the second sound wave from the synthesized signal to generate a third input signal (third signal generating means), and uses the third input signal as an interference signal generating device 23. And the target signal generator 24 (third signal output means). In the target signal generating device 24, a target signal is generated by the digital filter 29 and the equalizer 30 (target signal generating means). The target signal is output from the target signal generating device 24 to the second subtracting device 25 (target signal output means).

  The error microphone 12 that has detected the interfering sound wave during operation of the system 10 </ b> A outputs the interfering sound wave to the amplifier 20. The amplifier 20 amplifies the interference sound wave, and then outputs the interference sound wave to the A / D converter. The A / D converter converts the interfering sound wave into the second input signal, and then outputs the second input signal to the second subtractor 25. The second subtracting device 25 calculates an error signal by subtracting the target signal from the second input signal representing the interfering sound wave (error signal generating means), and outputs the error signal to the interference signal generating device 23 (error signal output). means). At this time, the level of the error signal, which is the difference between the target signal and the second input signal, is smaller than when the system 10A is activated.

  In the interference signal generation device 23, the adaptive control arithmetic device 28 updates the adaptive digital filter 27 using the error signal while taking into account the time delay of the secondary path and the transmission frequency characteristics. The interference signal generation device 23 performs a convolution operation on the adaptive digital filter 27 on the third input signal output from the first subtraction device 22 to generate an interference signal that minimizes the second input signal (interference signal generation means). The interference signal generation device 23 outputs the generated interference signal to the pseudo feedback signal generation filter 21 and the D / A conversion device (interference signal output means). The D / A converter converts the digital interference signal into an analog second sound wave, and outputs the second sound wave to the amplifier 19. The amplifier 19 amplifies the second sound wave and outputs the amplified second sound wave to the muffler speaker 13. The muffler speaker 13 generates a second sound wave in the internal space 16 of the duct 15.

  In this system 10A, the equalizer 30 can be adjusted during operation to set the frequency band and the volume of the frequency to values different from the initial settings, and the sound of the frequency band to be muted and the frequency band to be attenuated or amplified can be set. Sounds can be changed from those in the default settings. When the frequency band or volume is changed during operation of the system 10A, and the sound of the frequency band to be silenced or the sound of the frequency band to be attenuated or amplified is changed, the target signal generation device 24 uses the changed sound. A target signal is generated. This system 10A can freely change the sound of the frequency band to be muted and the frequency band to be attenuated or amplified by adjusting the frequency band and volume through the equalizer 30 during operation. In addition, since interference signals corresponding to them can be generated promptly, the sound in the frequency band to be silenced or attenuated or amplified during operation of the system 10A according to changes in the noise source 17, changes in the internal environment of the duct 15, and the like. The sound of the frequency band to be made can be freely modified.

  FIG. 3 is a diagram showing an example of the silencing effect (experimental result) by this system 10A, and FIG. 4 is a diagram showing another example of the silencing effect. In these drawings, the frequency characteristic of the internal space 16 of the duct 15 when the system 10A is OFF is indicated by a dotted line L1, the frequency characteristic of the internal space 16 when the system 10A is ON is indicated by a solid line L2, and the frequency characteristic of the target signal is indicated by a one-dot chain line. Indicated by L3. In these figures, the vertical axis represents the sound pressure level, and the horizontal axis represents the frequency. In the system 10A, the frequency characteristics shown in the drawings can be monitored in time series via the display, and the monitored frequency characteristics can be output via the printer. Further, the monitored frequency characteristics can be stored in the memory in time series.

  As an example of use of this system 10A, an equalizer is used to attenuate sounds in the frequency bands of 100 (Hz), 125 (Hz), and 160 (Hz) and bring the frequency characteristics of these frequency bands closer to the frequency characteristics of the target signal. After setting the frequency band and volume (sound pressure level) through 30, the system 10 </ b> A was activated. As a result, as shown in FIG. 3, sounds in the frequency bands of 100 (Hz), 125 (Hz), and 160 (Hz) were effectively attenuated, and approximated to the frequency characteristics of the target signal. As another example of the use of this system 10A, sound in the frequency bands of 200 (Hz), 250 (Hz), and 315 (Hz) is attenuated so that the frequency characteristics of these frequency bands are close to the frequency characteristics of the target signal. After setting the frequency band and volume via the equalizer 30, the system 10A was activated. As a result, as shown in FIG. 4, the sound in the frequency bands of 200 (Hz), 250 (Hz), and 315 (Hz) was effectively attenuated, and approximated to the frequency characteristics of the target signal.

  In this active silencing system 10A, the target signal generator 23 generates a target signal having a desired frequency characteristic after interference with the second sound wave, and the second subtractor 25 removes the target signal from the second signal. Therefore, the second signal output from the error microphone 12 can be approximated to the target signal, and the target signal generator 23 can arbitrarily and freely change the frequency characteristic at the position of the error microphone 12 to the expected frequency characteristic. Can be adjusted. The system 10A outputs only the frequency band to be silenced from the second signal to the interference signal generation device 23 as an error signal, and the interference signal generation device 23 generates an interference signal using the error signal and the third signal. An interference signal that interferes with only the sound in the frequency band to be silenced can be output to the silencer speaker 13 via the signal generator 23. This system 10A can arbitrarily and freely select the sound of the frequency band to be silenced from the noise propagating through the internal space 16 of the duct 15, and can surely cancel only the sound of the selected frequency band. Furthermore, the remaining frequency band remaining from the second signal can be arbitrarily and freely selected, and the volume of the sound in the remaining frequency band can be increased or decreased, so that part of the noise can be protected for speech privacy. Can be used as masking noise.

  In this system 10A, the first subtractor 22 generates a third input signal obtained by removing the pseudo feedback signal from the combined signal, and outputs the third input signal to the interference signal generator 23 and the target signal generator 24. The third signal output to the interference signal generation device 23 and the target signal generation device 24 does not include a pseudo feedback signal, and howling in the reference microphone 11 can be prevented. In the system 10A, the time delay until the first sound wave propagates from the reference microphone 11 to the error microphone 12 is corrected by the digital filter 29, and the transmission frequency characteristic of the first sound wave is used. The frequency band and sound pressure level of the third input signal can be adjusted via the equalizer 30 in a state where the characteristics of the noise transmission path to the microphone 12 are reflected in the third input signal. The target signal can be selected from the third signal via the equalizer 30 under the assumption that the signal is generated at the position of two sensors.

  In this system 10A, since the time series transfer characteristic until the second sound wave propagates from the muffling speaker 13 to the error microphone 12 by the digital filter 26 is used, the adaptive control arithmetic unit 28 is connected from the muffling speaker 13 to the error microphone 12. The adaptive digital filter 27 can be updated in time series using the error signal while taking into account the time delay and frequency characteristics of the secondary path up to and following changes in noise and target signals in the internal space 16. On the other hand, it is possible to reliably mute only the sound in the frequency band to be muffled at the position of the error microphone 12 that is the mute position. Since the sampling frequency of the system 10A is in the range of 3 kHz to 40 kHz, the second sound wave can be brought as close as possible to the sound having the opposite phase and same sound pressure of the sound to be silenced, and the frequency to be silenced using the second sound wave. The sound of the belt can be muted reliably.

  FIG. 5 is a configuration diagram of an active silencing system 10B shown as another example. The system 10B shown in FIG. 5 is different from that shown in FIG. 1 in that a delay correction device 31 is used instead of the digital filter 29, and the other configurations are the same as those of the system 10A shown in FIG. 1 are assigned the same reference numerals as those in the system 10A in FIG. 1, and the description of other components in the system 10B is omitted. In this system 10 </ b> B, the target signal generation device 24 is formed of a delay correction device 31 and an equalizer 30. The delay correction device 31 corrects the time delay S until the first sound wave propagates from the reference microphone 11 to the error microphone 12. The sampling frequency in this system 10B is in the range of 3 kHz to 40 kHz.

  An example of the silencing process in the system 10B will be described as follows. The first sound wave detected by the reference microphone 11 when the system 10B is activated is amplified by the amplifier 18, converted into a first input signal by the A / D converter, and output to the first subtractor 22. The first subtracting device 22 subtracts the pseudo feedback signal (0) from the first input signal to generate a third input signal (third signal generating means), and the third input signal is compared with the interference signal generating device 23 and the target. It outputs to the signal generator 24 (3rd signal output means).

  The delay correction device 31 that forms the target signal generation device 24 corrects the time delay S of the first sound wave from the reference microphone 11 to the error microphone 12 to reflect the time delay S in the third input signal. Then, the third input signal is output to the equalizer 30. The equalizer 30 converts the third input signal into a target signal having a preset frequency band and volume (sound pressure level) (target signal generating means), and outputs the target signal to the second subtracting device (target signal output) means).

  The first sound wave detected by the error microphone 12 when the system 10B is activated is amplified by the amplifier 20, converted into a second input signal by the A / D converter, and output to the second subtractor 25. The second subtracter 25 subtracts the target signal from the second input signal representing the first sound wave to calculate an error signal (error signal generator), and outputs the error signal to the interference signal generator 23 (error signal). Output means).

  In the interference signal generation device 23, the adaptive control arithmetic device 28 updates the adaptive digital filter 27 using the error signal while considering the time-series transmission characteristics of the secondary path. The interference signal generation device 23 performs a convolution operation on the adaptive digital filter 27 on the third input signal output from the first subtraction device 22, and generates an interference signal that minimizes the second input signal in time series (interference signal generation). means). The interference signal generation device 23 outputs the generated interference signal to the pseudo feedback signal generation filter 21 and the D / A conversion device (interference signal output means). The D / A converter converts the digital interference signal into an analog second sound wave, and outputs the second sound wave to the amplifier 19. The amplifier 19 amplifies the second sound wave and outputs the amplified second sound wave to the muffler speaker 13.

  The muffler speaker 13 generates a second sound wave in the internal space 16 of the duct 15. The second sound wave propagates from the speaker 13 toward the reference microphone 11 through the internal space 16 of the duct 15 and from the speaker 13 toward the error microphone 12 through the internal space 16. The second sound wave travels toward the error microphone 12 while interfering with the first sound wave, interferes with the sound of the frequency band to be silenced or the sound of the remaining frequency band, and cancels the sound of the frequency band to be silenced. Amplify or attenuate the sound of the remaining frequency band.

  During operation of the system 10B, a synthesized sound wave of the second sound wave and the first sound wave is detected by the reference microphone 11. In addition, an interference sound wave between the second sound wave and the first sound wave is detected by the error microphone 12. The synthesized sound wave is amplified by the amplifier 18, converted to a digital first input signal by the A / D converter, and output to the first subtractor 22. As in the system 10A of FIG. 1, the pseudo feedback signal generation filter 21 convolves the time series transfer characteristic (Hr) with the interference signal, generates a pseudo feedback signal (pseudo feedback signal generation means), and generates a pseudo feedback signal. Output to the first subtractor 22 (pseudo feedback signal output means). The first subtracting device 22 subtracts the pseudo feedback signal from the combined signal to generate a third input signal (third signal generating means), and the third input signal is used as an interference signal generating device 23, a target signal generating device 24, (Third signal output means). In the target signal generation device 24, a target signal is generated by the delay correction device 31 and the equalizer 30 (target signal generation means). The target signal is output from the target signal generating device 24 to the second subtracting device 25 (target signal output means).

  The error microphone 12 that has detected the interfering sound wave during operation of the system 10 </ b> B outputs the interfering sound wave to the amplifier 20. The interference sound wave includes sound in the remaining frequency band. The amplifier 20 amplifies the interference sound wave, and then outputs the interference sound wave to the A / D converter. The A / D converter converts the interfering sound wave into the second input signal, and then outputs the second input signal to the second subtractor 25. The second subtracting device 25 calculates an error signal by subtracting the target signal from the second input signal representing the interfering sound wave (error signal generating means), and outputs the error signal to the interference signal generating device 23 (error signal output). means).

  In the interference signal generation device 23, the adaptive control arithmetic device 28 updates the adaptive digital filter 27 using the error signal while taking into account the time-series transfer characteristics of the secondary path. The interference signal generation device 23 performs a convolution operation on the adaptive digital filter 27 on the third input signal output from the first subtraction device 22, and generates an interference signal that minimizes the second input signal in time series (interference signal generation). means). The interference signal generation device 24 outputs the generated interference signal to the pseudo feedback signal generation filter 21 and the D / A conversion device (interference signal output means). The D / A converter converts the digital interference signal into an analog second sound wave, and outputs the second sound wave to the amplifier 19. The amplifier 19 amplifies the second sound wave and outputs the amplified second sound wave to the muffler speaker 13. The muffler speaker 13 generates a second sound wave in the internal space 16 of the duct 15.

  Also in this system 10B, the equalizer 30 can be adjusted during operation to set the frequency band and the volume of the frequency band to values different from the initial settings, and the sound of the frequency band to be muted or attenuated or amplified. The sound of the frequency band to be made can be changed from those of the initial setting. When the frequency band or volume is changed during operation of the system 10B, and the sound of the frequency band to be silenced or the sound of the frequency band to be attenuated or amplified is changed, the target signal generation device 24 uses the changed sound. A target signal is generated.

  The effect of the active silencing system 10B is different from that of the system 10A of FIG. 1 because the digital filter 29 is replaced by the delay correction device 31 and is as follows. In this system 10B, since the time delay S until the first sound wave propagates from the reference microphone 11 to the error microphone 12 is corrected by the delay correction device 31, the time delay S between the reference microphone 11 and the error microphone 12 is corrected. The frequency band and sound pressure level of the third signal can be adjusted via the equalizer 30 in the corrected state, and the equalizer 30 is assumed to be in the state where the third signal is generated at the position of the reference microphone 11. The target signal can be selected from the third signal. The system 10B has the same effects as the system 10A of FIG. 1 in addition to the effects described above. However, the effects of the system 10A of FIG. 1 are used, and description of other effects of the system 10B is omitted.

  These illustrated systems 10A and 10B are used when the microphones 11 and 12 and the speaker 13 are installed in the internal space 16 of the air-conditioning duct 15 to effectively mute the noise in the duct 15, but these systems 10A and 10B are used. The use location of 10B is not limited to the internal space 16 of the duct 15, and the illustrated systems 10A and 10B can be used in a space other than the duct 15.

The block diagram of the active silence system shown as an example. The figure explaining a response characteristic. The figure which shows an example of the silencing effect (experimental result) by a system. The figure which shows another example of the muffling effect (experiment result) by a system. The block diagram of the active silence system shown as another example.

10A Active silencer system 10B Active silencer system 11 Reference microphone (first sensor)
12 Error microphone (second sensor)
13 Silencer speaker (second sound generator)
DESCRIPTION OF SYMBOLS 14 Controller 15 Air conditioning duct 16 Internal space 17 Noise source 18 Amplifier 19 Amplifier 20 Amplifier 21 Pseudo feedback signal generation filter 22 1st subtraction apparatus 23 Interference signal generation apparatus 24 Target signal generation apparatus 25 2nd subtraction apparatus 26 Digital filter (2nd digital) filter)
27 Adaptive Digital Filter 28 Adaptive Control Computing Device 29 Digital Filter (First Digital Filter)
30 Equalizer 31 Delay Correction Device

Claims (7)

A first sensor disposed at a predetermined distance from the noise source, and a first sensor disposed at a predetermined distance from the noise source with the first sensor interposed therebetween, and is generated from the noise source and propagates through a predetermined space. A second sound wave generator that generates a second sound wave that interferes with the sound wave in the space, a second sensor that is spaced apart from the first sensor by a predetermined dimension across the second sound wave generator, and the second sensor An interference signal generating device that generates an interference signal to be a sound wave and outputs the generated interference signal to the second sound wave generating device, wherein the first sensor is a synthesized sound wave of the first sound wave and the second sound wave. Are detected in time series, and the detected synthesized sound wave is output as a first signal, and the second sensor detects in time series the interference sound wave generated by the interference between the first sound wave and the second sound wave, and detects it. Active interference wave output as a second signal In the system,
The active silencing system receives a first signal from the first sensor and a pseudo feedback signal generation filter that generates a pseudo feedback signal that estimates a second sound wave that reaches the first sensor from the second sound generator. Meanwhile, a first subtraction device that receives a pseudo feedback signal from the pseudo feedback signal generation filter and generates a third signal obtained by removing the pseudo feedback signal from the first signal, and the third signal from the first subtraction device. And a target signal generator for adjusting the frequency characteristics of the third signal to convert the third signal into a target signal having a predetermined frequency band and sound pressure level, and receiving the second signal from the second sensor A second subtracting device that receives the target signal from the target signal generating device and generates an error signal obtained by removing the target signal from the second signal; It includes,
The target signal generation device wants to mute or attenuate a first digital filter that simulates a time-series transfer characteristic from the first sensor to the second sensor and an arbitrarily selected frequency band of the third signal. The volume of the frequency band is set to minus, and an equalizer is set to set the volume of the frequency band to be amplified or left of the arbitrarily selected frequency band of the third signal to be plus ,
The first digital filter uses a time series transfer characteristic in a noise transmission path from the first sensor to the second sensor estimated in advance, convolves the time series transfer characteristic with the third signal, and performs the first calculation. The noise transmission path is corrected by correcting a time delay until the first sound wave propagates from the sensor to the second sensor and matching the frequency characteristic of the first sound wave with that of the first sound wave at the position of the second sensor. The third signal is output to the equalizer in a state in which the characteristic of is reflected in the third signal,
The active silencer system, wherein the interference signal generation device generates the interference signal using a third signal received from the first subtraction device and an error signal received from the second subtraction device. A first sensor disposed at a predetermined distance from the noise source, and a first sensor disposed at a predetermined distance from the noise source with the first sensor interposed therebetween, and is generated from the noise source and propagates through a predetermined space. A second sound wave generator that generates a second sound wave that interferes with the sound wave in the space, a second sensor that is spaced apart from the first sensor by a predetermined dimension across the second sound wave generator, and the second sensor An interference signal generating device that generates an interference signal to be a sound wave and outputs the generated interference signal to the second sound wave generating device, wherein the first sensor is a synthesized sound wave of the first sound wave and the second sound wave. Are detected in time series, and the detected synthesized sound wave is output as a first signal, and the second sensor detects in time series the interference sound wave generated by the interference between the first sound wave and the second sound wave, and detects it. Active interference wave output as a second signal In the system,
The active silencing system receives a first signal from the first sensor and a pseudo feedback signal generation filter that generates a pseudo feedback signal that estimates a second sound wave that reaches the first sensor from the second sound generator. Meanwhile, a first subtraction device that receives a pseudo feedback signal from the pseudo feedback signal generation filter and generates a third signal obtained by removing the pseudo feedback signal from the first signal, and the third signal from the first subtraction device. And a target signal generator for adjusting the frequency characteristics of the third signal to convert the third signal into a target signal having a predetermined frequency band and sound pressure level, and receiving the second signal from the second sensor A second subtracting device that receives the target signal from the target signal generating device and generates an error signal obtained by removing the target signal from the second signal; It includes,
The target signal generation device corrects a time delay until the first sound wave propagates from the first sensor to the second sensor, and among the arbitrarily selected frequency bands of the third signal And an equalizer that sets the volume of the frequency band that is desired to be muted or attenuated to minus and the volume of the frequency band that is desired to be amplified or left to be amplified among the arbitrarily selected frequency bands of the third signal,
The delay correction device corrects the time delay of the first sound wave from the first sensor to the second sensor, and reflects the third signal in a state in which the time delay is reflected in the third signal. Output to the equalizer,
The active silencer system , wherein the interference signal generation device generates the interference signal using a third signal received from the first subtraction device and an error signal received from the second subtraction device . In the frequency band in which the volume is set to be positive in the equalizer, the amplified target signal is output from the target signal generation device to the interference signal generation device via the second subtraction device, and the sound at the position of the second sensor is output. In the frequency band in which the second sound wave generator outputs a second sound wave so that the sound volume is set to be negative in the equalizer, the attenuated target signal is transmitted from the target signal generator to the second subtractor. 3. The active device according to claim 1 , wherein the second sound wave generating device outputs the second sound wave so as to attenuate the sound at the position of the second sensor that is output to the interference signal generating device via Mute system.   The interference signal generator uses the adaptive digital filter for generating the interference signal that minimizes the second signal in time series, and the adaptive digital filter using the error signal received from the second subtractor. The active silencing system according to any one of claims 1 to 3, wherein the active silencing system is formed of an adaptive control arithmetic device that updates the system.   The interference signal generation device includes a second digital filter that simulates a time-series transfer characteristic from the second sound wave generation device to the second sensor, and the adaptive control arithmetic device includes a transfer function in the second digital filter and The active silencing system according to claim 4, wherein the adaptive digital filter is updated from an error signal received from the second subtractor.   The active silencing system according to any one of claims 1 to 5, wherein a sampling frequency in the active silencing system is in a range of 3 kHz to 40 kHz.   The active silencing system according to claim 1, wherein the space is an internal space of an air conditioning duct.

Images